Published 4:00 am, Sunday, January 14, 2007

It wasn't all that long ago that black holes existed only in the realm of theory, a space- and mind-bending musing of Albert Einstein, who posited the existence of objects in the universe so dense that even light could not escape them. Even after scientists began to accept several decades ago that these exotic and powerful objects were not the stuff of science fiction, they still knew virtually nothing about them.

Over the past 10 years, however, black holes have moved to the center of the world of astrophysics, leading to a steady flow of discoveries that have begun to answer, or at least better describe, some of their mysteries.

It's not hard to see why. Recent discoveries include the likelihood that black holes, some as massive as a billion suns, exist at the center of all galaxies and may have played a key role in forming them. Researchers have also found that black holes arising from burned-out and collapsed stars generally emit bursts of super-powerful gamma rays as well as enormous jets of particles moving at 175 million mph, 26 percent of the speed of light.

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And when it comes to proving, or disproving, Einstein's theory of relativity, there is no other laboratory nearly as compelling.

"Black holes are extremely exotic beasts, objects where time stands still at some point, where space-time curves, where nothing inside can be seen," said Christopher S. Reynolds, an astrophysicist and black hole researcher with the University of Maryland. "The fact that they actually exist remains amazing to me."

As understood now, black holes are objects so dense, so brimming with gravitational force, that they constantly suck matter in -- and never let it out. Researchers now believe there are millions of them, some just a few miles across, some as wide as the solar system.

Although black holes are to some extent defined by their death grip on all kinds of light, astronomers now detect them by the extremely bright light and other radiation that emanates from around a hole as it swallows a nearby star or galactic gases. Detectible bursts of gamma rays, the highest-energy explosions in the universe, also shoot out as a star explodes and collapses into a black hole.

NASA's fleet of orbiting satellites, most especially the Hubble Space Telescope, the Swift gamma-ray satellite and the Chandra X-Ray Observatory, have been drivers for these discoveries, providing information and images never before available to illuminate the behavior of black holes.

The Swift satellite, which was launched in 2004, provided information for the most recent discovery, one that identified an apparently new and different variety of black hole.

Generally, black holes form or grow larger when the core of a massive star collapses or when an existing black hole collides with a tiny but very high-density neutron star, itself the unimaginably dense remains of a burned-out star. Instruments on Swift have been able to monitor the bursts of high-energy gamma rays that accompany the formation of a black hole -- an event some astronomers compare to a baby's birth scream -- and scientists are learning how and why a black hole was created based on how long the bursts last.

What was believed to be a relatively neat division between long and short bursts was challenged, however, by a gamma-ray burst recorded by Swift in June from a galaxy 1.6 billion light-years away. The burst was quite long, yet it had none of the characteristics of a massive star collapse generally associated with long bursts. For now, astronomers are calling it a hybrid black hole.

"This is brand new territory; we have no theories to guide us," said Neil Gehrels, lead investigator for Swift at the NASA Goddard Space Flight Center in Greenbelt, Md. "It's stretching our models to the biggest extent of any burst so far."

This process of identifying black holes, which Gehrels said may well number in the hundreds of millions, is one step on the path toward understanding them better. Other researchers have sought to measure the two known characteristics of a black hole -- its mass and the speed at which it spins.

Using data from NASA's Rossi X-Ray Timing Explorer satellite, a team from the Harvard-Smithsonian Center for Astrophysics determined last year that one particularly large black hole was spinning at more than 950 times per second.

The fast pace of recent discoveries has set the stage for a new generation of sophisticated observatories, which together have been dubbed Beyond Einstein by NASA. While several of the proposed satellites would study black holes, the most pertinent is Constellation-X, a four-telescope X-ray observatory planned to be 100 times as powerful as any instrument now available for probing X-ray radiation and black holes.

Constellation-X could revolutionize the understanding of black holes and the very nature of gravity, but budget cuts, to some extent prompted by NASA's ambitious plans for renewed manned space exploration, have put the $1.5 billion-to-$2 billion project on hold.

The agency has held out the promise that some Beyond Einstein funding will be available in the 2009 budget, and last year NASA asked the National Research Council to convene an expert panel to recommend which of five major Beyond Einstein projects is most valuable and ready to go. A selection is expected by fall.